T-Wave alternans (TWA) are beat-to-beat fluctuations in amplitude, polarity, or shape of a T-wave of an electrocardiogram (ECG) or electrogram (EGM) signal, and may be used to predict sudden cardiac death (SCD). Studies have shown TWAs to have a very high negative predictive value. For example, 30% of patients indicated for an implantable cardioverter defibrillator (ICD) produce negative TWA result, and only 2-5% of these patients have a VT/VF episode. In contrast, TWA positive patients have a high likelihood of experiencing a VT/VF episode, e.g., approximately a five times higher chance compared to a negative patients.
Visible macroscopic TWA present in an ECG measured with surface electrodes have been associated with imminent onset of dangerous ventricular arrhythmias, and in some cases, may leave limited time to intervene. Microvolt level TWA can be used to assess subtle changes in repolarization that occur far in advance of arrhythmia. Microvolt level fluctuations in the T-wave are not detectable by the unaided human eye, but can be revealed with computer analysis. Measurement of surface ECG TWA requires preprocessing the input signal to reduce noise and/or analyzing 128 or more sinus beats. TWA values recorded from intracardiac EGMs of implantable cardioverter defibrillators (ICDs) and TWA recorded from surface ECGs have substantial concordance.